1972 Mercury 800 Thunderbolt Fuel Prob.

[Gomer50]

I am having alittle problem.My motor quit running today,checked spark(good),new spark plugs.Checked gas (good) but here's where I am not sure,I took off gas line from fuel pump to carbs and pumped the bulb and gas comes out of fuel pump.If I crank the motor over no fuel comes from the fuel pump Am I thinking that it is time to rebuild fuel pump?? Any advise would greatly be appreciated.Just to let you folks know this forum has been so much help.Could not ask for more.

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

I am new to the outboard world and am guessing that when you crank your motor gas is supposed to come out of the fuel pump which in my case is not happening.Is this a correct assumtion?:

 

[Fuzzytbay]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Sounds like you need to rebuild the pumps. At cranking, some fuel should pulse out of the pumps.

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Thanks for responding,Pump works fine now.I now need to rebuild the carbs.One of my floats will not move (stuck).Any Idea's where to buy these or do they come in a rebuild kit.Its amazing how all of a sudden this all starts to go bad.Ran fine until I started using 97 octane fuel instaed of 90.

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

I rebuilt the carbs and it runs once again,but I dont understand I also purchased NGK BUHW(1) Spark plugs and it doesnt like them at all.Will start but dies shortly afterwards.I put the old autolite plugs back in and it runs like a champ??? Any Suggestions......

 

[jake505]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

thats weird maybe one of the plugs is bad. not very common but i have seen it before

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Thanks for the reply.It starts and maybe give it 45 seconds and it starts sounding like its starving and soon dies and wont start up again.I put in the old autolite fires right up idles good,revs up good.Shut it off fires right back up.I would hate to see that I bought a couple of no good plugs.Like you said I put a new set of plugs in my truck and ran like crap couple of bad plugs.I think its time to upgrade with a newer motor..Thanks again

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Well I am back again with maybe needing some more advise.Took the boat out last night to test under load well not so good.Could not get a hole shot died.Tried to set the idle mix screws and Idle screw for the distributor.To get it running half way decent it would ldle @ 10 before TDC and idle mix screws out 2+ turns.Idle good hole shot good but would starve out shortly after,Pump up the bulb and do it again.Seems like its having a hard time getting gas.Battery
finally died out from repeat starting.I am thinking that maybe my carb/fuel pump rebuild did not go so well.What do you folks think???

 

[jake505]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

when you rebuilt your carbs did you clean them out real good and make sure there was no little crap floating around in them that could clog them up and that all the ports are clear?

 

[Fuzzytbay]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Did you try pumping the bulb, while it was dieing out? It could be the fuel pump still. Another thought is there is an air leak on the carb rebuilds. 2+ turns out seems too much. One reason for the extra rich mixture would be a slight air leak.

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Thanks for the reply,I will take the carbs and fuel pump back off and try again not that I dont have anything better to do LOL...
Oh just a thought is it possible to hook up an electrical fuel pump with maybe 3 to 4 pound psi and bypass the original ?????

 

[jake505]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

yea i wouldnt put an electric fuel pump on because sometimes the pressure can be to much and screw up your carbs though i have done it before with out a problem besides you can pick up a rebuild kit for those fuel pumps on ebay for like 9-10 bucks each which is way cheaper and easier then trying to put in a fuel pump. usually all you need is a pin hole leak in one of the diaphrams in of one of the pumps and it wont work right mechancally not much to go wrong because they use the pulsing air flow of the engine to pump so just rebuild them and make sure your carbs are good and clean and it should get you going.

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Thank for the info I thinks thats the way I will go.(Rebuild).I did notice I had a small leak I forget whats its called but its where th fuel line from the bulb that goes to the quick disconnect under the cowl.I notice that when I had diconnected the fuel line from engine and pumped the bulb,gas was coming from the whatever its called.Fixed that but havent had a chance to see if that made a difference.Anyway I really appreciate your help...

 

[jake505]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

yeah you might want to change the fuel fitting on the engine end too I had one that was causing me a headache too it was leaking and I think it as restricting fuel to because when I changed it it help a lot but I had put on all new fuel lines to so maybe that was the problem but I think 40 years out of a connector and rubber fuel lines is pretty good lol

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

I am replacing as I go if its not one thing its another lol..I bought this boat prob. about 8 months ago.Motor looked good boat ok took it out boat leaked. did patch and gel coat the bottom.No more leaks now the motor in giving me a run for my money.I think the only thing this motoer has going for it, has good compression.Hopefully I'll get this thing in descent running condition before I retire...

 

[Gomer50]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

Okay need just alittle more advice,after rebuilding carbs,fuel pump.replaced bulb,fuel lines.Starts pretty good, Idles pretty good.Hole shot is great but now comes the kicker.At 4200 rpm it runs decent,at WOT throttle it spits and sputters, gerking and wants to go just cant do it.I had put new spark plugs in it from NGK.And these did not work at all so put back the old autolite plugs.I am wondering if these plugs are fouling at higher RPM.Thankd guy's I read here alot and am very thankfull.Everytime I take it to the lake in the hopes that I have finally got it right BUT NO.Thank the Lord I have got patience.

 

[Fuzzytbay]

Re: 1972 Mercury 800 Thunderbolt Fuel Prob.

If you have done any work on the carbs, adjusted the max timiing, or idle set screw, or max throtle stop, then you now need to do a link n sync to the motor, or risk serious damage. Here is a link to how to do a link n sync for you motor.
http://forums.iboats.com/showthread.php?t=168855